1. Field of the Invention
The present invention relates to optically active liquid crystal materials and liquid crystal compositions containing the optically active liquid crystal materials.
Throughout the specification, the "liquid crystal material" includes not only materials which exhibit the liquid crystal phases by themselves, but also materials which are usable as ingredients in liquid crystal compositions although not detected to exhibit the liquid crystal phases by themselves.
2. Prior Art Statement
Liquid crystal displays have advantages that they are actuated by low electric voltage, that they consume only little electric power, that they are fabricated into thin layer forms, and that they are relatively light in weight. Making use of these advantages, liquid crystal display devices are applied for use in desk electric calculators, watches and televisions. Nematic liquid crystals have hitherto been widely used as the materials for such display elements. However, nematic liquid crystals are detrimental in that the response speeds thereof are low, i.e. in the order of several tens milliseconds. In order to avoid the disadvantage of the nematic liquid crystals, it has been proposed to use ferroelectric liquid crystals in the display system, for example, by N. A. Clark et al., "Submicrosecond Bistable Electro-Optic Switching in Liquid Crystals", Appl. Phys. Lett., 36, 899 (1980). In the display system proposed by this prior art reference, the chiral smectic C liquid-crystalline phase (hereinafter referred to as "Sc* phase") of a ferroelectric liquid crystal is utilized. It is required that the ferroelectric liquid crystal materials used for such purpose exhibit the Sc* phase in a wide temperature range, have high spontaneous polarization and low rotational viscosity. However, the known ferroelectric liquid crystal compounds are detrimantal in that they exhibit the Sc* phase in only a narrow temperature range and that the spontaneous polarization thereof is low. Particularly, the compounds having azomethine groups are inferior in chemical stability and thus cannot be applied for practical use. Accordingly, there is an increasing demand for a ferroelectric liquid crystal which has chemical stability and high spontaneous polarization and which exhibits the Sc* phase in a wide temperature range.
In order to apply ferroelectric liquid crystals in a practical electro-optic switching element, it is required to control the Sc* temperature range and the electro-optic characteristics thereof, such as spontaneous polarization, viscosity, helical twist and tilt angle, within proper ranges. To satisfy the requirement, it has been tried to use a mixture containing a compound having high spontaneous polarization or inducing high spontaneous polarization, a compound having a low viscosity, compounds having reverse helical twists, and compounds which exhibit the Sc or Sc* phase in a wide temperature range. It becomes thus necessary to investigate a number of materials to find out the optimum condition for the appropriate characteristics, such materials to be investigated include:
(1) Materials having or inducing high spontaneous polarization; PA1 (2) Compounds which can unloose or release the helical twist by the addition thereof in a small quantity; and PA1 (3) Liquid crystal compounds exhibiting the smectic C phase in a wide temperature range.
It is thus demanded to find a number of liquid crystal materials having various properties and being capable of constituting the liquid crystal compositions.